With an ever increasing emphasis on sustainability and the environment, there has been a corresponding increase in research directed toward developing polymeric materials derived from or comprising renewable biologically derived components.
Much of the research to date in developing such polymeric materials has focussed on utilising naturally occurring bio-polymers such as starch. Starch is attractive in that it is derived from renewal resources (i.e. plant products), readily available and relatively inexpensive. However, the mechanical properties of starch in its native form are quite poor compared with those of petroleum derived (i.e. “synthetic”) polymers.
The mechanical properties of starch can be improved by melt mixing it with a plasticiser such as a polyhydric alcohol so as to form TPS. However, the improved mechanical properties of TPS still remain relatively poor compared with those of petroleum derived polymers.
Starch or TPS in its own right is therefore not considered a viable alternative to petroleum derived polymers.
In attempt to derive the benefits of starch and petroleum derived polymers, a considerable amount or research has been directed toward developing blends of these two polymer materials. However, combining relatively hydrophilic starch with petroleum derived polymers, which are typically hydrophobic, to produce blends with good mechanical properties has proven difficult in practice. In particular, melt processing starch or TPS with petroleum derived polymers generally results in the formation of polymer blends having a multi-phase discontinuous morphology. Such morphologies are typically unstable and exhibit high interfacial tension, the like of which often has a negative impact on at least the mechanical properties of the resulting polymer blend.
Nevertheless, useful blends of starch or TPS and petroleum derived polymers have been developed. For example, polyester/TPS blends have been shown to exhibit good mechanical properties and can be formulated so as to be fully biodegradable. However, due to the hydrolytic sensitivity of the polyester matrix their application can be limited, for example such blends are not generally suited for melt-recycling.
Polyolefin/TPS blends (e.g. polyethylene/TPS blends) have also been developed. Due to the non-hydrolytic sensitivity of the polyolefin matrix, such blends are at least expected to be more suited to recycling than their polyester/TPS counterparts. However, the inherent incompatibility between the highly hydrophobic character of polyolefins and the hydrophilic character TPS has proven problematic in attaining blends with useful properties.
U.S. Pat. No. 6,605,657 discloses a method of preparing a polyolefin/TPS blend in which a relatively moisture free TPS is prepared in a first extrusion unit and then as a melt is combined with a polyolefin melt prepared in a second extrusion unit. The resulting blend is said to exhibit good mechanical properties. However, at least the method by which the blend is made is not without its shortcomings.
While polymer blends comprising both TPS and petroleum derived polymers have been developed, and opportunity remains to address or ameliorate one or more disadvantage or shortcoming associated with such blends and/or the methods by which such blends are prepared, or to at least provide a useful alternative polymer blend and/or method for preparing it.